Telegraph receiver



1950 E. M. s. MCWHIRT-ER ETAL 2,498,695

TELEGRAPH RECEIVER 3 Sheets-Sheet 1 Filed April 25, 1947 gven t rv- /z/%W Alto ney Feb. 28, 1950 MOWHIRTER ETAL 2,498,695

TELEGRAPH RECEIVER I Filed April 25, 1947 Z Shets-Sheet 2 Patented Feb.28, 1950 TELEGRAPH RECEIVER Eric Malcolm Swift McWhirter, Roland HarrisDunn, and Peter William Lennox, London, England, assignors toInternational Standard Electric Corporation, New York, "N ."Y.

Application April 25, 1947; Serial No. 144,010 In Great Britain February119, 1.946

Section ;1, Public Law 690, August 8, 1946 Patent expires February '19,1966 Claims. 1 This invention relates to telegraph receivers. Atelegraph receiver mustin general comprise a means for timing theperiods occupied by the Successive elements of the .code. This ;isgenerally done by means of rotating mechanical parts and great accuracyof manufacture is necessary for these parts.

According to one feature of the present invention there is provided areceiver fora telegraph code comprising. a time base derived by electricdischarge services for timing the in,- tervals occupied by the elementsof said code, and means for determining the kind of elements received ineach of said intervals.

According to another feature of the invention there is provided areceiver fora telegraph code comprising means for generating an evenlyspaced series of electric impulses, means .ior timing the intervalsoccupied by the elements of said code by the intervals betweensaidimpulses and means for determining the ,kind .of element received ineach of said intervals.

The invention will be described as. applied to the reception of startstop teleprinter codes and as so applied a receiver according to onefeature of the invention comprises an electric impulse generator forgenerating, ,an ,evenly spaced series of impulses, ,means responsive tothe start element of a code signal for starting said impulse generatorinto. operation, and means for timing the intervals occupied :by :theelements of said code by the intervals betweensaid impulses and meansfor determining the :kindpf element being received at the ,middlewofeach of said intervals.

Preferably the receiver according to the in.- vention comprises a chainof gas filled electric discharge devices which are caused to'be renderedconducting sequentially by sequentialimpulses and in the case of areceiverfor start. stop teleprinter codes the electric impulse generatorthat is started into operation by the receipt of the start element of acode delivers an impulse to the electric discharge device correspondingto the start element of the codeaf-ter a delay-equal to half the periodof time allotted to the start element. Each electric discharge devicecauses the succeeding device to become conducting when the next impulseis applied so that these successive devices become conducting at themidpoint of the successive elements and the time interval to be occupiedby the signal is determined by the chain of devices.

The invention has been devised primarily for use in the electricsignalling system claimed :in

copending U. S. patent application Ser. No. 744,009. In thissystem theclosure of contacts individual todifferent pieces of apparatus .at oneor more substations causes the transmission of pre-determinedteleprinter codes individual to the particular contacts to a supervisorystation where a device individual to the contacts at the transmitter isoperated upon the receipt .of the predetermined code signal. Theinvention accordingly provides a receiver tor predeterminedtelegraphcodes. comprising for each said code a chain of .gasfilled.electricdischarge devices, one for each element of the code, means forsetting the successive devices .of each chain so as to cause .each to.be rendered conducting only .upon the simultaneous occurrence vof thepreceding device in the chain .being conducting and an elementiof, therequired kind being received means for causing..-successi ve elements ofthe code to be applied to successivedevices in the chain and meansoperative. upon the last device in the chain :becoming conducting .to.cause the receipt of the code to'become operative.

It is preferred that the codes .used in such a system should be chosen@sothat the earlier occurring elements areas far as possible common to aplurality of codes and :the several chains of electric discharge devices.are combined to the extent that devices representing elements common to.-a plurality of codes, are provided and are connected so as to ,becommon to the like plurality-pf chains ofsuchdevices.

This principle is readily extended to provide reception of the fullnumber of codes possible.

Thus for astart-stop teleprinter codeusing five code elements, thirtytwo different combinations are possible. There are thus thirty twochains of electric discharge devices required but the number of suchdevices is reduced by arranging one such device for the start elementwhich is common: to all the combinations. This device is connected to ,apair of devices forthe second element ,of the code, one for a markingelement and one for a spacing element. The start hecomes conducting .inresponse to the start element and impresses on each device of the saidpair of a potential tending to make it conducting. When the first codeelement is received it impresses a second potential on that one of thepair that corresponds to the-kind of element. The two potentials addedtogether make one of the pair conducting. Each device of this pair isconnected to twofurther devices for the next element and so on ingeometrical progression. In this way any particular code receivedprogresses along only one chain of electric discharge devices and onlyone device corresponding to the last element of the code combinationbecomes conducting and indicates the receipt of that particular codecombination.

Although this completed chain of electric discharges in one sense timesthe telegraph code, it is preferred to use a separate means forthispurpose and in the preferred form this separate means consists of aseparate pilot chain of electric discharge devices rendered conductingsequentially by the application thereto of an evenly spaced series ofimpulses.

The invention will be better understood from the following descriptiontaken in conjunction with the accompanying drawings in which:

Figs. 1 and 2 together form one circuit diagram, Fig. 1 showing thepilot chain of tubes and a receiver for one predetermined code and Fig.2 the circuit of the impulse generator and the telegraph relaysreceiving signals from the line;

Fig. 3 shows diagrammatically how tubes of the receiver may be arrangedfor a certain number .of code combinations; and

Fig. 4 shows diagrammatically the arrangement of tubes of the receiverfor a complete 5- unit code receiver.

Referring to the drawings and first to Fig. 1 there are shown two trainsof three electrode gas-filled cold-cathode tubes. The first or pilotchain consists of seven tubes I 'I, one for each element of the code.These tubes are connected each to the next in the chain in a mannersimilar to that shown in U. S. Patent No. 2,421,005, but modified in animportant respect.

All the tubes are similar so that only one need be referred to indetail. Tube I comprises an anode 8, a control electrode 9' and a coldcathode II]. Inorder to render the tube conducting the gap between thecontrol electrode 9 and the cathode I!) must be ionised and a positivepo-' tential must be applied to the anode 8 with respect to the cathode.So long as this potential is 130 volts or more, the minimum ionisationcurrent produced by a voltage of about 70 volts between controlelectrode 9 and cathode I0 is sufiicient to cause current to flowbetWeen anode 8, and cathode IEI. As the potential applied between anodeand cathode diminishes below 130 volts, the ionisation current betweencontrol electrode 9 and cathode III and consequently the potentialapplied between them, that is needed'to render the tube conducting,increases rapidly.

When the tube I is conducting, current flows through resistances II andI2 between cathode I0 and earth. In U. S. Patent No. 2,421,005 theresistances between the cathode and earth were so proportioned'as togive a voltage drop of about 35 to 40 volts thereacross and the cathodewas connected directly to the control electrode of the succeeding tube(such as tube 2). The source of potential I3 for the anodes of tubes I'1 is derived ultimately from the mains and if the mains voltage falls,the voltage applied to the anode of tube 2 falls and the nominal voltageapplied to its control electrode from the cathod circuit of tube I alsofalls.

In the present circuit therefore, resistances II and I2 are proportionedto give a voltage drop of twice that previously provided between cathodeand earth and the junction point of these resistances is connected (overa resistance I4 as previously) to the control electrode of tube 2. Thefall in the nominal voltage applied tov the control electrode 9 of tube2 due to a given fall in mains voltage is then less than before.Moreover the voltage drop between the source l3 and the anode of tube Iis reduced so that tube I is more easily extinguished by tube 2 becomingconducting. The voltage-drop across tube I when conducting is about 70volts so that the anode of tube I is about volts above ground potential.As the anode of tube 2 is connected directly to the anode of tube I andthe cathode of tube 2 is at earth potential when the tube is notconducting, the voltage applied between the anode and cathode of tube 2is held, even if the mains voltage varies, above the value at whichminimum potential difference between control electrode and cathode willcause the tube i ecome conducting.

In the initial condition of the circuit of Figs. 1 and 2, tube I isconducting and a potential difference is impressed from its cathodecircuit between the control electrode and cathode of tube 2. Thispotential difference is about 35 volts, which is insufficient to causethe tube to become conducting, the potential difference required forthis purpose being about '70 volts.

The incoming line I5 Fig. 2 is connected over windings of telegraphrelays I6 and IT to ground. As in the normal, non-signalling conditionof line I5 marking potential is impressed thereon at the transmittingstation, the contacts I8 and I9 of these relays are in marking position.The cathode of tube I Fig. l is connected over a conductor 20 to themarking contact of contacts I9 and thus through a condenser 2I andground. Condenser 2I is thus kept charged.

Fig. 2 shows the circuits of an impulse generator and the means by whichit is started into operation. The impulse generator comprises twopentodes 22 and 23 connected in well known manner as a multivibrator.The anode potentials are supplied from a source stabilised by means of aneon-tube stabiliser 24. The normal grid potentials are adjusted bymeans of potentiometers 25 and 26, the sliding contact on 25 serving toadjust the frequency and that on 26 the relative times of operation ofthe two tubes. The impulses are derived from the grids of pentodes 22and 23, these grids being connected over respective condensers to thegrids of triodes 21, 28 so that the changes of potential on the grids ofpentodes 22 and 23 being thus differentiated to produce positiveimpulses on conductors 29 and 38 connected to the anodes of thosetriodes.

The multivibrator is held in non-operating condition by the cathode oftube 22 being normally connected to positive potential. A threeelectrode cold cathode tube Si is normally conducting and currenttherefore flows through resistance 32 connected to the cathode thereof,the positive end of this resistance being connected to the cathode ofpentode 22. There is a second three electrode cold cathode tube 33,which is normally non-conducting, the anode of which is connected over acondenser 34 to the anode of tube 33.

The spacing contact of contacts I 9 is connected to earth over theprimary of a step-up transformer 35, the secondary of which is connectedto the control electrode of tube 33. When therefore the start element ofa teleprinter code combination is received and contacts I8 and I9 ofrelays I6 and I! change over to spacing position, condenser 2I isdischarged through the primary of transformer 35. The resultantpotential across the secondary of this transformer 35 is impressedbetween control electrode and cathode of tube 33 and is sufiicient toionise the gap between those anes oes electrodes and :theitube 33becomes conducting. The iIItEFCOHIIGCtlOD- OTI'thB anodes :of-etubesMxand 33 through condenser 34Jres1I1tsin tube :3l becomingnon-conducting andthe disappearanceof the positive bias on the cathodepipentode 22.

The -positivebias on. the cathode. of pentode 22 haswprev-iously held.this .apentode from passing current-and the multivibrator .isthereforenormal ly' in the position .:.in lWhiCh: anode current .inpentode .23 .a maximum. .Whenzthe positive potentialon the cathode:of-tube 2'2.:is removed thelgridpf that tube becomes positivewithrespect to=the cathode and the tube l 22 passes :eur-rent and theanode current rapidly :rises *to :a maximum and remains so until the:condenservconnecting the'anode of pentode 22.to:the. grid of pentodet23has discharged. Pentode 23 then :passes current and there .is a suddendrop in-the potential of the gridof pen-toder-2 2. r Thisrsudden :dropinypotential lis differentiated .:by. condenser 3fixand .a positivepulse created :on .conductor..;29 connected to the lanodez oftube 121..The .periodiof the multivibrator may belsetsso that this :pulse occursmilliseconds after :the commencement of -its operation and thus atitheim'id-point of fthe periodof milliseconds .ior :the :start elementof theincoming signal (for a teleprinter. signalling speed of 50 bands).

Conductor: 29.15 connected to a conductor .3"! which .;.in turn :isconnected over an individual condenser resistance combination to thecontrol electrodes :of :tubes'2 ,1. The value of the potential thusimpressed on these control electrodes .is winsuflicient tosionise thegap between controlelectrodeand cathode-of anyof the tubes, but :added:tothe potential- 'imptessedon the control electrode-of .tube 2" fromthecathode .circuit :of :tube l :is -suflicient :tocause tube Ii-tobecome conducting.

When tube 2 becomes conducting a potential is impressed from its cathodocircuit uponthe control electrode of tu'be (and ailso upon the controlelectrode of another tube to bere'ferred to resent1 and upon the nextoccurrence -of an-impulse onconductcu' 3! i. -e. after a-period-of 20"milliseconds, tu'be-3 beeomesconducting. in turnimpressesa potentialupon tube t. in this way tubes 3, '4,"-'5 ii-and "1 become conducting onthe "occurrence of successive impuls'es.

It shouldbe noted-that "the anodes' of .all tl re tubes I 'l areconnected to positivepoterrtial over a common resistance 'ttyvvhich-is-of' such value thatwv'hen a tube suchas "tube 2' becomes conducting.the potential uponthe anode'ofany other tube,such as l,that'isat'that'm'oment-conducting is "depressed "below "the value necessarytokeep thetube conducting whenronl a'verylow potential'is applied across"the gap between control electrode and cathode. The consequence is thatwhen any tube in the chain -l ...""l "becomes conducting the. preceding'tube in the chain becomesnon-conducting.

Thetubes 1| '1 correspond .to. the different elements. of. a.startestopfive unit ,teleprinter code. Tube 1 .is apilot tube, normallyconductingand corresponding to the stop. e'lementlof the code. Tube 2corresponds tothe .start element of the code...and tubes v3 to .-the.code elements. The tubes .2 Sl -operated sequentiallyby ,the impulses onconductor 31, time ltheintervalsoccupied by the elements of the :code,becomin conducting at the mid-points of the elements allottedi'tothecode.

The cathode circuit of tube It deaconnected over conductor-:39 tto :the.2901117130.]. electrode. sofas. :col-d cathode tube 40 (Fig. 2) whichcontrol electrode also :connected :toaconduetor The impulse generatorapplies; anxlimpulse JtO..:-CQD-d1l0t.l' 123i ten milliseconds after.each impulse aplll'iedto to the connection/between the anodes :of tubes.34

and 33 over aoondenser.3:4,;tube.33:becomes nonconducting. Anotherieftectlof tube l becoming conducting istotimpressa positivepotentialupon the. cathodeuof pentodeizoand so st p he-impulse generator. Tube.1551318 zconnected so as. :to be self-extinguishing.

The secondary 59f transiormerrdil .-.:is.q.con.-nect.ed over .a;conductor '42. to the-control {electrode of tube letFig; 1') ;and theimpulse delivenedwhen tube 4:10 becomes conductingalso (causes tube i tobecomeconducting. JT-Ilube V1'! is thereby :rendered non-conductinSoiarnorreference hasibeemmade to {the efi fict oizthereoeipt of the'.cod.e lelementscof;a.signal. Fig. 1 :shows {a receiver for .a,particular modecombination. receiver consists of five tubes 43 14.1.correspondin to the :fiwe mode-elements of rayfive unitteleprinterpode. The anodes-40f all these tubes .are co-nnecte'dtogether, and through a resistanceASto a p0SitWe.-S0ll1tGeA9. .illhecathode circuit .ofaitube is -.connected .-to the control electrode oftube A3 and thencathode-circuit of each :of the tubes .-:43 -46.is-;connected-.to-the controlrelectrode-of the nenttubeein the-"chain inthe same .manne as theltubes I l areconnected together.

. Thezeontrol eleptrodesioftubes 43 pillar-.6 connected throughindividualcondensersand resistances to either of two conductors 15.0out! r which in turn are connected-to the spacing and marking :contactsmespectively of relay .15. The connections between.thercontrolielectrodes of the tubes :43 47 and conductors-:50and-iluare preset ,in accordance withra particular code combinatiominthe case. shown, .mark, space, space, space, mark. The operation; is asfollows:

Tube- 2 .is made conducting-when the-,startelement of a teleprintersignal .is received, as .described above, and a .potential is therebyimpressed not only between. control electrode and cathode of tube.3-asdescribed above, but also between controlelectrode and cathodeof.tubedt. The second impulseon conductor ,29 occurs ,at an interval of20. millisecondsafterthefirstimpulse, i. e. in the middle of theinterval allotted to the first code elementpf .the received signal.Conductor 2.9 is connected to the moving tongue ofcontacts t8 and ifatthis momentama-rking impulse is being .rec,eived,.s,o that contacts i8.are in marking position, this impulse is applied over conductoril to thecontrol electrode of tube-43. The potentialdue to this impulse, added.to the potential impressed vfrom the cathode circuit of tube.2 causestube .43tobecome.conducting. The cathode circuit .oftube .43 impressesapotential on'the-control electrode .of tube .44. ..If, on .theoccurrence of the next impulse on conductorifl, contacts .18 are*inspacing position, the-potential of this impulse .is. applied iovercondudtorfifl .to the ;control electrode of tubes, .and' .tube 44becomes 'conducting'and tube'43 is made nonconducting.

If, on the other hand, contacts l8 were in spacing position on theoccurrence of the second impulse on conductor 29, tube 43 would not havebeen made conducting. In this case tube 43 would not be made conductingupon any subsequent impulse. Also if tube 43 has been made conductingbecause the first code element is a mark, but the second code element isalso a mark so that on the occurrence of the third impulse on conductor29 contacts l8 are in marking position, tube 44 would not be madeconducting. Tube 44 will however be made conducting on the occurrence ofa subsequent spacing element forming part of the code combination.

It will be clear that if the code combination received be that for whichtubes 43 to 41 is set these tubes will be made conducting in succession,but that if any other code combination be received, tube 47 will nothave been made conducting by the time the code combination has come toan end and the creation of impulses from the multivibrator 22, 23 hasceased. If the correct code combination has been received and tube 41has become conducting, current flows from a point in the cathode circuitof tube 4! through a rectifier 52 in the forward direction to charge acondenser 53 in such sense as to apply positive potential upon the gridof a triode vacuum tube 54 whereby a relay 55 connected between thecathode of tube 54 and ground is operated. Relay 55 closes its contacts56 to operate a meter 51 or other indicator. The rectifier 52 is shuntedby a resistance 58 which allows the condenser 53 to discharge in abouthalf a second.

An additional cold-cathode tube 59 is provided having its anodeconnected to the anodes of tubes 43 41. The control electrode of tube 59is connected to the conductor 42, so that when, at the end of the periodallotted to the receipt of a start-stop teleprinter code tube i is madeconducting by an impulse from the secondary of trans-- former 4| overconductor 42, as above described, tube 59 is also made conducting. anodeconnection ensures that any of the tubes 43 41 that may then beconducting is made non-conducting. Thus the tubes 43 41 are ready forthe receipt of a further code combination, whether the code combinationfor which they were set had, or had not been received. Tube 59 will berendered. non-conducting during the receipt of a subsequent codecombination if any of the tubes 43 41 then becomes conducting.

It will be clear that as many sets of tubes such as 43 4'! may beprovided as there are code combinations to be received, each set beingconnected up so that, upon the receipt of the correct code combination arelay such as 55 will be operated.

It is however possible to choose the codes to be operative at thereceiver in such manner as to reduce considerably the number of electricdischarge devices required. This may be done by choosing the codes sothat the earlier occurring elements are as far possible common to aplurality of codes. The several chains of electric discharge devices arethen combined to the extent that devices representing elements common toa plurality of codes are provided and connected so as to be common tothe like plurality of chains of such devices.

Fig. 3 shows diagrammatically how this can be done for four and eightcodes respectively. Each of the circles represents a cold-cathode tube.

The common Those tuba that have control electrodes con nected to themarking conductor I, Figs. 1 and 2, are shown as black circles, thosethat have control electrodes connected to the spacing conductor 50 areshown as open circles. One arrangement of tubes for four codes is shownabove the dashed line. It will be noted that the first three tubes 60,6|, 62 arranged for mark, space, space as the first three elements arecommon to all four codes. For the fourth element two tubes are provided63 and 64, the former for a spacing element, the latter for a markingelement. The cathode circuit of tube 62 is accordingly connected to thecontrol electrodes of both tubes 63 and 64. Similarly for each of thetubes 63 and 64 there is a pair of tubes 65, 66 or 61, 68 for the finalelement of the code.

The portion of Fig. 3 below the dashed line shows the additional tubesneeded to extend the arrangement to the receipt of eight codes, and fromthis it will be clear how the arrangement could be extended for thereceipt of any number of codes permitted by the number of code elementsused, that is 92 for a five-unit code. The letters placed in Fig. 3opposite each tube for the final element of the code are the lettersrepresented by the particular chain of devices that ends in thatelement.

In this way a complete teleprinter receiver may be constructed, as shownin Fig. 4 which is composed of electric discharge devices, apart fromthe two receiving telegraph relays I6 and Il and the relays such as 55which can be arranged either to efiect the printing of a character or toperform any desired supervisory or indicating function. There are norotating parts and mechanical wear is reduced to a minimum. The starttube 2 is shown in Fig. 4 and from this figure the connections of theremaining tubes will be clear.

When a code combination is received, only that chain of gas-filled tubescorresponding to the particular code combination is completed. Otherchains are left in a condition in which one of the tubes other than theend tube of the chain is conducting when the receipt of the codecombination comes to an end. All tubes that are thus left conducting arecaused to become non-conducting when the tube 59 becomes conducting, sothat the equipment is ready to receive a subsequent code.

It should be noted that the multivibrator of Fig. 2, may be set by meansof the sliding contact on resistance 26 so that the relative periodsduring which it is in the respective conditions of stability are equaland this adjustment has been assumed in the foregoing description. Inthis condition of adjustment an impulse occurs on conductor 29 after thelapse of 10 milliseconds (assuming that the slider on resistance 25 isset for a total period of 20 milliseconds). The sliding contact onresistance 26 allows however of the adjustment of the initial period tosuit distortion in the line [5, this adjustment corresponding to thatwhich is termed orientation adjustment with mechanical teleprinters. Theimpulses on conductor 29 follow one another after equal periods of time,the length of which depends on the frequency of the multivibrator,determined by the adjustment of the sliding contact on resistance 25.What is claimed is:

1. A receiver for predetermined telegraph codes comprising for each saidcode a chain of gasfilled electric discharge devices, one chain for eachelement of the code; an electric impulse generator for generating aseries of regularly repeated impulses, means for applying impulses fromsaid generator to successive devices ofs'aid chains upon receipt of acode element the required kind, means to apply an impulse fr m apreceding device in a chain simultaneouslylwith said last named means,said successive devic"; in said chains adapted to be rendered conductingin accordance with the corresponding code lement, means operative uponthe last devic'e'j in an operated chain becoming conducting to indicatethe receipt of the code element, and means connected to each chain forrestoring the devices thereof to non-conducting condition at the end ofthe period allotted to the receipt of the cod 2. A receiver as claimedin claim 1 comprising means for timing impulses from said impulse"generator to occur at the mid-points of the period allotted to thereceipt of successive elements of the code.

3. A receiver as claimed in claim 2 in which a pilot chain of gas-filledelectric dischargedevices is provided, one for each element of. thecode, comprising means for rendering said j ,devices conductingsuccessively, for periods equal to the periods of the successiveelements of the code and means operative at the end of the periodallotted to the receipt of a code combination/for stopping said impulsegenerator and for restoring the receiver into a condition to receive afurther code combination. s

4. A receiver as claimed in claim 3, in which said means for generatingelectric impulses or said electric impulse generator comprises amultivibrator, and means for differentiating the changes of potentialupon an electrode of said multivibrator to produce a series of impulses.

5. A receiver as claimed in claim 4 in which said pilot chain comprisesone of said discharge devices normally in a conducting condition, meansfor impressing a first positive potential from said device upon a startdevice of said chain corresponding to the start element of a signal,disabling means for normally holding said impulse generator in onecondition of stability, tele- 10 graph relays connected to respond toincoming signals, means operative upon the movement of contacts of saidrelays in response to the start element 01' a signal to render saiddisabling means inoperative, means for deriving from said impulsegenerator impulses timed to occur at the midpoints of the start elementand succeeding elements, means for applying the first of said impulsesto render said start device conducting and subsequent impulses both torender successive devices of said pilot chain conducting and to renderconducting in succession devices of a chain of gas-filled devices presetto respond to elements of a particular kind, means responsive to thelast device of said last mentioned chain becoming conducting to recordthe corresponding character and means responsive to the joint occurrenceof the last device of said pilot chain becoming conducting and of animpulse from said impulse generator in anti-phase to said firstmentioned impulses for restoring said disabling means and for renderingnon-conducting any device of any of said pre-set chains that may beconducting and all other devices except the one corresponding to thestop element of said pilot chain.

ERIC MALCOLM SWIFT MCWHIRTER.

ROLAND HARRIS DUNN.

PETER WILLIAM LENNOX.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 897,662 Roberts Sept. 1, 19081,068,869 Edwards July 29, 1913 2,099,065 Holden Nov. 16, 1937 2,411,441Leroy Nov. 19, 1946 2,412,642 Wilherson Dec. 17, 1946 2,430,547 AndersonNov. 11, 1947 FOREIGN PATENTS Number Country Date 512,827 Great BritainSept. 29, 1939

